The present invention relates to a process of separation of hafnium from zirconium by molten salt electrolysis which produces zirconium trichloride and zirconium tetrachloride with a low hafnium content suitable for production of reactor grade zirconium containing about 100 ppm of hafnium.
Zirconium ore containing generally 2.about.4 wt% of hafnium. In conventional technical processes of separation of hafnium from zirconium, one is a solvent extraction process using methyl isobutyl ketone as a solvent for thiocyanic acid solution (J. W. Ramsey, W. K. Whitson Jr.: Production of Zirconium at Y-12, AEC Report Y-817, Oct. 12, 1951). And the other is an extractive distillation process in which vapors of zirconium tetrachloride and hafnium tetrachloride are absorbed selectively by countercurrent circulation of a fused solvent consisting of aluminium trichloride and potassium chloride and are separated continuously from each other in a distilling column (French Pat. No. 7340395, Japanese Patent Publication No. 20279 of 1978). And U.S. Pat. No. 2797485 discloses a process of separation of hafnium from zirconium tetrachloride concerned with reduction of zirconium tetrachloride to zirconium trichloride using a reducing agent and with distillation of hafnium tetrachloride.
However, in the above mentioned solvent extraction process, the recovery of secondary reaction products is expensive owing to its large equipment scale and its use of a large quantity of chemicals. In the extractive distillation process, the running cost is lower than that of the solvent extraction process owing to its simple process and its possible recovery and reuse of the fused solvent, but the cost of equipment is high because of its tall and stout distilling column required. Moreover, the reduction process is not yet industrialized owing to its difficult recovery of the reducing agent used.
On the other hand, as mentioned above, zirconium ore contains generally 2.about.4 wt% of hafnium. As the standard free energies of chlorination reaction of both oxides are slightly different and hafnium tetrachloride is more stable than that of zirconium, a hafnium content in the chlorides is almost the same as that of the ore. Both tetrachlorides of zirconium and hafnium are starting materials for production of zirconium and hafnium metals. As the thermal neutron absorption cross section of hafnium is very large, a hafnium content in zirconium used for a nuclear reactor must be suppressed to about 100 ppm.